1. Field of the Invention
The present invention relates to a dielectric isolation substrate for a semiconductor device and a method of manufacturing the dielectric isolation substrate and, more particularly, to a composite substrate for isolating a high-breakdown-voltage element from other elements and a method of manufacturing the composite substrate.
2. Description of the Prior Art
The structure of the conventional dielectric isolation substrate and a method of manufacturing the dielectric isolation substrate will be described below with reference to FIGS. 1A to 1E. An example of the dielectric isolation substrate is described in International Symposium on Power Semiconductor Devices and ICs, '92, pp. 316-321. After V-shaped grooves are formed in a monocrystalline silicon substrate 1, an n.sup.+ -type buried layer 2 is formed on the surface of the monocrystalline silicon substrate 1 (FIG. 1A).
Thereafter, a silicon dioxide film 3 is formed on the surface of the n.sup.+ -type buried layer 2 (FIG. 1B).
A polycrystalline silicon film 4 is formed on the surface of the silicon dioxide film 3 and then abraded and polished to an abrasion surface 10a to have a predetermined thickness (FIG. 1C).
Thereafter, the abrasion surface 10a adheres to a monocrystalline silicon support substrate 5. In this case, the monocrystalline silicon support substrate 5 having a thickness of 300 to 500 .mu.m is used (FIG. 1D).
The monocrystalline silicon substrate 1 is polished to obtain island-like monocrystalline silicon films 1a (FIG. 1E). Note that FIG. 1E shows a structure obtained by turning the structure in FIG. 1D upside-down.
Since this conventional dielectric isolation substrate has a large amount of warpage and many crystal defects, the yield in the subsequent step decreases. In case of a large amount of warpage, when the polycrystalline silicon film 4 filled in the V-shaped groove in FIG. 1E or a trench is to be polished, the polycrystalline silicon film 4 has different polishing rates at the peripheral and central portions of the substrate. For this reason, the island-like monocrystalline silicon films 1a have different thicknesses, respectively, and the breakdown voltages of the high-breakdown-voltage elements vary. In addition, when many crystal defects are present in an element, the defects decrease the lifetimes of carriers or cause leakage.